A conventional driving device for charging and discharging a piezoelectric actuator to expand and contract the piezoelectric actuator includes a charging circuit and a discharging circuit, as disclosed in JP 2005-16431A for example. In the charging circuit, electric power is supplied from a DC power source through a charging switch having MOSFET to a series circuit of an inductor and the piezoelectric actuator. The discharging circuit is connected to the series circuit in parallel. In the discharging circuit, electric charges charged in the piezoelectric actuator are discharged through a discharging switch having MOSFET.
In this driving device, when a driving signal is input from an external device to the driving device, the charging switch is repetitively turned on/off while the discharging switch is in the turned-off state, thereby charging the piezoelectric actuator and thus expanding the piezoelectric actuator. Then, when no driving signal is input, the discharging switch is repetitively turned on/off while the charging switch is in the turned-off state, thereby discharging the piezoelectric actuator and thus contracting the piezoelectric actuator.
That is, during the charging period, the piezoelectric actuator is charged in stepwise manner according to the following procedure. That is, the charging switch is turned on while the discharging switch is turned off, whereby charging current flows from the DC power source through the charging circuit into the piezoelectric actuator. Thereafter, the charging switch is turned off so that the charging current (flywheel current) flowing by means of the energy accumulated in an inductor is made to flow from the negative side of the piezoelectric actuator to the positive side thereof through a parasitic diode of MOSFET used as the discharging switch.
On the other hand, during the discharging period, the piezoelectric actuator is discharged in stepwise manner according to the following procedure. That is, the discharging switch is turned on while the charging switch is turned off to make the discharging current flow from the positive side of the piezoelectric actuator to the discharging circuit. Thereafter, the discharging switch is turned off so that charging power is regenerated to the DC power source by a flyback voltage generated by the energy accumulated in the inductor.
In this type of driving device, a desired charge amount is charged into the piezoelectric actuator by controlling the amount of charges per unit time which are supplied from the DC power source.
That is, when the driving signal is input to this driving device, the charging switch is turned on, and then turned off at the time point when the integration value of the charging current flowing into the piezoelectric actuator reaches a predetermined target value. Thereafter, the charging switch is turned on when the charging current reaches a smaller predetermined value than the target value concerned. This turn-on/off operation is repeated, thereby charging the piezoelectric actuator.
Since the electrostatic capacitance of the piezoelectric actuator varies in accordance with temperature, the current amount of charging current flowing into the piezoelectric actuator at the charging time varies in accordance with surrounding conditions under which the piezoelectric actuator is used.
When the electrostatic capacitance of the piezoelectric actuator is varied, the amount of current flowing from the DC power source into the piezoelectric actuator when the charging switch is turned on is varied, and thus the energy accumulated in the inductor at that time also varies. Therefore, the time required until the discharging of the energy from the inductor is completed under the state that the charging switch is turned off is also varied.
Therefore, during the charging period, the timing at which the charging switch is turned on is varied in accordance with the variation of the electrostatic capacitance of the piezoelectric actuator, and thus the variation amount of charges accumulated in the piezoelectric actuator (that is, the expansion rate at which the piezoelectric actuator expands per unit time) is varied in some cases.
For example, in a case where the conventional driving device is used to control the fuel injection of an injector so that fuel injection is started at the time when the piezoelectric actuator expands by a predetermined amount, the timing of the fuel injection of the injector is varied, and thus it is impossible to perform high-precision fuel injection.